Traveling-wave tube automatic gain control



June 26, 1956 D. A. WATKINS TRAVELING-WAVE TUBE AUTOMATIC GAIN CONTROL Filed March 11. 1954 INVENTOR.

IIIIICIIIII ///r 454mm TRAVELING-WAVE TUBE AUTOMATIC GAIN CONTROL Dean Allen Watkins, Menlo Park, Califi, assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application March 11, 1954 Serial No. 415,572

Claims. (Cl. 179-171) This invention relates to traveling-wave tubes and more particularly to a traveling-wave tu-be incorporating main and auxiliary electron streams for automatically controlling the gain of the tube.

In some high frequency receivers, it is desirable to employ an intermediate-frequency amplifier incorporating several traveling wave tube amplifiers connected in cascade. In a receiver of this type, the average amplitude of the output signals must be maintained substantially constant irrespective of the amplitude of the received signals. This applies especially to radar receivers wherein local ground clutter may produce large amplitude signals while distant targets produce very small amplitude signals. Automatic gain control apparatus is employed in radar receivers to produce signals of substantially constant amplitude, these signals, in turn, producing uniformly lighted spots or targets on the screen of a cathode ray viewing tube. In this manner, the large amplitude ground clutter or other signals do not saturate the phosphor of the viewing screen, thereby blotting out smaller amplitude target signals appearing contiguous to them. Thus it is seen that it is desirable to incorporate automatic gain control in the traveling wave tube amplifiers used in this type of amplifier.

In present day traveling-wave amplifiers, amplification of a traveling-wave is obtained by directing an electron stream having a slightly higher velocity than the wave along a path disposed in the field of the wave. Amplification of the wave is effected by the transfer of some of the energy of the stream electrons to the wave by means of mutual interaction therebetween. The stream voltage and current may be adjusted, however, so that the wave is attenuated considerably more than it could be amplified. An electron stream having a certain predetermined velocity and current can produce attenuations as large as from 50 to 70 decibels whereas the same stream could only effect a gain from 20 to 30 decibels. See, for exam ple, an article entitled On the operation of the travelingwave tube at low level, by R. Kompfner which appears on page 283 in the Journal of the British Institute of Radio Engineers for August-September 1950, published in London, England.

The automatic gain control circuit of the present invention incorporates a traveling-Wave tube having both a main and an auxiliary electron stream. The velocity of the auxiliary electron stream is adjusted to either attenuate or amplify the signal to be amplified by the main stream of the tube. It is preferable that the velocity of the auxiliary stream be adjusted to attenuate the signal inasmuch as a stream of predetermined current can effect considerably more attenuation than gain. Also, for small signals the extent to which the signal is amplified or attenuated is proportional to the current of the stream. Thus, in the present invention, a circuit is provided to produce a voltage proportional to the magnitude of the output signal and to employ this voltage to control the current in the auxiliary stream to minimize the variations in the amplitude of the output signal.

nite States Patent 0 F It is desirable to employ an auxiliary electron stream so that its velocity may be adjusted to highly attenuate the signal and thus make the automatic gain control considerably more sensitive to changes in the output signal amplitude. An additional advantage of employing an auxiliary stream is that the saturation level of the main stream producing the amplification of the signal is not disturbed.

It is, therefore, an object of this invention to provide an improved automatic gain control for wave-type amplifier tubes.

Another object of the invention is to provide a gain control circuit for a traveling-wave tube amplifier for producing an output signal of substantially constant amplitude without affecting the saturation level of the main electron stream.

Still another object of the invention is to provide sensitive means for controlling the gain in a wave-type ampli fier tube.

A. further object of the invention is to provide an improved method of automatically controlling the gain .of a wave-type amplifier.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing, constituting a part of this specification, in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. l is a diagrammatic sectional view of an embodiment of the invention with associated circuitry.

Referring now to the drawing, there is shown in Fig. 1 an embodiment of the disclosed microwave oscillator which comprises a traveling-wave tube 10 including an input matching cavity 12 having a coaxial input cable 14 and an output matching cavity 16 with a coaxial output cable 18. An envelope 20, which provides the evacuated chamber of traveling-wave tube 10, consists of a long cylindrical structure which has an enlarged portion at the left extremity as illustrated in the drawing. Within this enlarged portion at the left extremity, there are located electron guns 22 and 24 for developing a main and an auxiliary electron stream, respectively. Gun 22 comprises a cathode 25 with a heater 26, a control grid 28, and an accelerating anode 30. Heater 26 is connected across a source of potential, such as battery 32, the negative terminal of which may be connected to a cathode 25 in addition to being referenced to ground, as shown. Grid 28 is connected to cathode 25. Anode 3%) is connected to an adjustable contact arm 34 of a source potential 36, the negative side of which is referenced to ground. A potential of the order of 1000 volts with respect to ground is representative of the potential normally applied to anode 30.

Gun 24 comprises a cathode 38 with a heater 49, a control grid 42, and an accelerating anode 44. Heater 40 is connected across a source of potential, such as battery 46. Cathode 38 is maintained at a variable potential that is positive with respect to ground in order that the velocity of the auxiliary stream may be adjusted to a predetermined velocity less than that of the main stream. This voltage is impressed on cathode 38 by means of a connection therefrom to a tap 48 of a potentiometer 50 which is connected across a battery 52, the negative terminal of which is connected to ground. Anode 44 of gun 24 is maintained at the same potential as anode 30 of electron gun 22 by a connection thereto.

A solenoid 54 is axially positioned symmetricallyabout the complete length of envelope 20. An appropriate direct current is maintained in solenoid 54 by means of a potential source, such as a battery 56, so as to produce a magnetic field of the order of 1000 gauss running axially along the length of the traveling-wave tube. The purpose of this magnetic field is to keep the electron streams focused or constrained along parallel paths coextensive with the elongated portion of envelope 20.

Proceeding along from the electron guns 22 and 24 in the direction of flow of the electron streams, there are positioned successively about the paths of the electron streams, a matching ferrule 58 connected by a lead 60 to a helix 62 which is, in turn, connected by a lead 64 to a matching ferrule 66. A collector 68 is positioned at the end of the path so as to intercept the stream electrons.

Helix 62, which serves as a slow-wave circuit for traveling-wave tube 10, preferably has an inner diameter substantially equal to the inner diameter of ferrules 58 and 66 so that the stream electrons can be made to pass as close to helix 62 as possible without being intercepted by the latter. A material such as tungsten is suitable for making helix 62, the principal requirement being that it retain its form, especially with respect to its pitch and diameter. In accordance with the present invention, the voltage applied to helix 62 may be adjustable or variable as this voltage produces the static electric potential throughout the active length of envelope 20 through which the electron streams flow and, hence, determines the average velocity of each electron stream Within helix 62. This voltage may be applied by means of a connection from ferrule 58 to an adjustable tap 70 on source 36. The magnitude of the potential applied to helix 62 may be of the order of from 500 to 2500 volts with respect to ground.

As previously mentioned, helix 62 is connected to ferrules 58 and 66 of leads 60 and 64, respectively. Leads 60 and 64 are located parallel to the electric fields excited within matching cavities 12 and 16. Matching cavity 12 has the configuration of a rectangular toroid with a concentric collar 72 disposed about matching ferrule 58 and a slot opening adjacent to lead 60. An opening 74 in the end plate of cavity 12 facing the left end of helix 62 allows the full length of lead 60 to be energized and, in addition, decreases the tendency of the electric field produced by the potential on the cavity from disturbing the flow of the electrons in the streams. Cavity 16 is similarly shaped, having a corresponding concentric collar 76 arranged about matching ferrule 66 and an opening 78 facing the right end of helix 62. A center conductor 82 of coaxial cable 14 extends through an aperture in the annular wall of cavity 12 and is connected to concentric collar 72 while the outer conductor of cable 14 is bonded to the periphery of the aperture. Likewise, a center conductor 84 of coaxial cable 18 extends through an aperture in the annular wall of cavity 16 and is connected to concentric collar 76 while the outer conductor of cable 18 is bonded to the periphery of the aperture in the same manner as before. Cavities 12 and 16 are fabricated with an inner surface composed of highly conductive material and are broadly resonant so as not to limit the frequency of operation. The configuration, shown and described for the cavities 12 and 16 in the drawing, provides suitable matching from helix 62 to coaxial cables 14 and 18 over a range of frequencies such as, for example, from 2000 to 4000 megacycles per second.

Due to the fact that traveling-wave amplifiers are broadband amplifiers, it is difiicult to obtain a proper impedance matching for all frequencies at the output. A resistive coating 80, which may be of carbon black, is applied on the outside of envelope 20 about the center turns of helix 62 for the purpose of attenuating waves which may be reflected from the output end of tube because of an impedance mismatch. Resistive coating thus decreases the tendency of the tube to break into oscillation. Forward waves traveling along with the stream electrons are not appreciably attenuated since they are propagated partially by the stream. It is to be noted that there are numerous methods of attenuating an electromagnetic wave and that the method described is merely for the purpose of illustration.

The stream electrons are intercepted by collector 68 at the opposite extremity of envelope 20 with respect to electron guns 22 and 24. Collector 68 is sealed to envelope 20 so as to have a large surface external to the evacuated chamber for heat dissipation purposes and may include fins to aid in conducting away the heat that is dissipated by the stream electrons when collected. Accordingly, collector 68 is preferably fabricated from a metal having good electrical and heat conducting properties such as, for example, copper or silver. A potential of the order of 30 volts positive with respect to that applied to helix 62 is applied to collector 68 in order to prevent secondary electrons which may be produced by the stream electrons impinging on its surface from reaching helix 62 or ferrule 66. This potential is applied by means of a connection from collector 68 to the positive terminal of source 36.

A feedback circuit 86 for tube 10 is connected from center conductor 84 of the output coaxial cable 18 to grid 42 so as to control the current in the auxiliary stream produced by electron gun 24. Circuit 86 comprises an input load resistor 88 and an output resistor 98 connected from the anode and cathode, respectively, of a diode 90 to ground. A capacitor 96 is connected in shunt across resistor 98 to smooth the output voltage of the circuit. The time constant of the combination of capacitor 96 and resistor 98 should be substantially larger than the pulse width employed with amplifier 10. The particular time constant employed is a compromise, that is, a very short time constant decreases the sensitivity of the receiver whereas a long time constant fails to accomplish the desired automatic gain control. The output voltage of circuit 86 is impressed directly on grid 42 of electron gun 24 by means of a connection thereto from the cathode of diode 90. Circuit 86 is coupled to the output of the traveling-wave tube 10 by the connection from output coaxial cable 18 through a capacitor to the anode of diode 90.

In the operation of amplifier 10, an input signal to be amplified is applied through input coaxial cable 14 to input cavity 12. The input wave in flowing along the exposed portion of conductor 82 within cavity 12 excites an electromagnetic field within the cavity. This electromagnetic field induces a corresponding voltage on lead 60 connecting ferrule 58 to helix 62 to launch a travelingwave along the helix 62. Interaction between the main electron stream and the traveling-wave results in a transfer of energy from the stream to the wave, causing it to grow or increase in magnitude. The auxiliary stream velocity is preferably maintained at a value to effect maximum attenuation of the wave as set forth in the aforementioned article by R. Kompfner, and average auxiliary stream current is maintained at a fraction of that of the main stream current. The wave, being propagated along helix 62, is then attenuated in proportion to the magnitude of the auxiliary stream current.

At the end of helix 62, the amplified electromagnetic wave, in flowing along lead 64 connecting helix 62 to ferrule 66, excites an electric field in cavity 16. This electric field induces a corresponding signal on center conductor 84 of coaxial cable 18. This output signal may then be utilized; a portion of the energy constituting this wave, however, is diverted to circuit 86 which rectifies the output signal to develop a direct-current control voltage proportional to the magnitude of the signal. This control voltage is then applied to control grid 42. By virtue of the auxiliary stream velocity, the wave propagated by helix 62 is attenuated in proportion to the current in the auxiliary stream. As the gain of tube tends to increase, the output signal increases, which results in a more positive control voltage which, in turn, is applied to control grid 42 of gun 24 which causes the auxiliary stream current to increase. The amount of this increase in auxiliary stream current is adjusted so that the attenuation of the wave is increased only to the extent that there is substantially no change in the average amplitude of the output signal. The converse applies in the event that there is a decrease in the amplitude of the output signal from the tube. Thus, self-compensation of gain variations is accomplished and the average amplitude of the output signal is maintained substantially constant.

What is claimed as new is:

l. A wave-type amplifier tube circuit capable of producing an output signal having a substantially constant amplitude, said circuit comprising a wave-type amplifier tube including means for propagating a signal wave along a path at a predetermined velocity substantially less than the velocity of light, the cross-sectional area of said path at any point along said path including first and second substantially mutually exclusive parallel portions, means for producing a first electron stream, means for directing said first electron stream along the first portion of said path at a velocity to effect constructive interaction be tween said first electron stream and said signal wave, thereby amplifying said signal, means for producing a second electron stream having less current than the current of said first stream, and means for directing said second electron stream along the second portion of said path at a velocity capable of producing a change in the amplitude of said signal wave in proportion to the current of said second stream; and means responsive to the amplified signal for controlling the current of said second electron stream in proportion to the magnitude of the amplified signal to produce an output signal of substantially constant amplitude.

2. The wave-type amplifier tube circuit as defined in claim 1 wherein said second electron stream is directed along the second portion of said path at a velocity to cause the electrons of said second stream to attenuate said signal wave in proportion to the current thereof.

3. A wave-type amplifier tube circuit capable of producing an output signal of substantially constant amplitude, said circuit comprising a traveling-wave tube including a conductive helix for propagating a signal wave along a path at a velocity substantially less than the velocity of light, said path including first and second substantially mutually exclusive parallel portions, means for producing a main electron stream, means for directing said main electron stream along the first portion of said. path at a velocity to cause the electrons of said main stream to constructively interact with said signal wave thereby amplifying said signal, an electron gun for producing an auxiliary electron stream, said electron gun having a grid for controlling the current in said auxiliary stream, and means for directing said auxiliary electron stream along the second portion of said path at a velocity capable of causing said auxiliary stream to simultaneously attenuate said signal wave in proportion to the current of said auxiliary stream; and means responsive to the amplified signal connected to the grid of said electron gun for controlling the current of said auxiliary stream in proportion to the magnitude of said amplified signal to produce an output signal of substantially constant amplitude.

4. The wave-type amplifier tube circuit as defined in claim 3 wherein said means responsive to the amplified signal for controlling the current of said auxiliary stream includes means for converting said amplified signal into a corresponding direct-current voltage, said direct-current voltage being impressed on the grid of said electron gun by said connection thereto.

5. The wave-type amplifier tube circuit as defined in claim 4 wherein said means for converting said amplified signal into a corresponding direct-current voltage com prises a diode having a plate and a cathode, an input load resistor and an output resistor connected from said plate and said cathode, respectively, to a point of substantially fixed potential, and a smoothing capacitor connected in shunt across said output resistor, said amplified signal being impressed across said input load resistor to produce said direct-current voltage across said smoothing capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 

